Current detection technology can be classified into two major categories: (1) solid-state devices based on silicon or wide bandgap semiconductors, and (2) a combination of photoemissive device (e.g., photocathode), a gain component (e.g., a microchannel plate), and an electron detector. FIG. 1 summarizes the quantum efficiency of some conventional. UV detectors.
In the first category, an immense investment has been made in silicon visible imagers in order to produce detectors with very low noise, low dark current, and very large formats (e.g., larger than 4 k·4 k pixels). In addition, new techniques such as lateral gain CCDs and low noise CMOS sensors are being developed so that silicon sensors continue to improve and become viable for photon counting applications. It is of course possible to use silicon sensors directly as a UV detector. The problem is that silicon sensors are almost never solar blind, a feature that is required for use in most interplanetary applications. In fact, the UV sensitivity must be incorporated as an “add-on” since conventional devices usually begin to cut off for wavelengths shorter than about 400 nm. The UV response is therefore almost always worse than the visible. While it is possible to make a camera system solar blind by using optical. filtering, the UV/visible rejection ratio desired by most planetary applications places enormous burdens on the filler designer. The less than unity UV/visible response-ratio of silicon sensors makes a challenging job even more difficult.
New materials such as gallium nitride (GaN), silicon carbide, and even diamond are being formed into detector arrays. Usually an array of diodes is made and is hybridized to a CMOS readout. Additionally, monolithic imagers with transistors made from the host or substrate material. exist at least in concept. These materials offer the promise of direct solar-blind UV imagers and research into their development is important. However, realistically, it will be more than a decade before monolithic imagers (not photocathodes) in these new materials can provide performance equivalent to silicon imagers.
The available detector chosen for most imaging applications has been a microchannel plate (MCP)-based imager. MCP-based detectors use a solar-blind photocathode that absorbs photons (UV, visible or near-IR) and ejects the photoelectrons into the vacuum. The free electrons are then collected by an MCP that is made from an array of tubes that are on the order of ten micrometers in diameter. Electrons entering these tubes are accelerated by high fields and they free electrons from the tube walls in each collision, creating an electron multiplier effect. The MCP provides electron signal. gain while the tube structure confines the electrons and preserves the spatial. information. Electron bursts generated by the MCP are read out by various schemes such as multi-anode arrays. Another scheme for readout consists of a phosphor for conversion of electrons to photons and a silicon imager (CCD or CMOS) sometimes coupled through fiber optic faceplates. The latter readout scheme is often referred to as an image intensifier.
While MCP-based imagers have been used successfully, they have significant drawbacks, such as low QE, limited resolution, small format size, limited number of pixels, high voltage requirements, and difficulty of fabrication in sealed-tube configuration. The QE in practice is ˜10% for wavelengths longer than 150 nm. Attempts to incorporate opaque near UV photocathodes on MCPs have faded. Resolution appears to be limited to 25-40 microns by fundamental. charge cloud variance in MCPs that cannot be predicted a priori. Also, format size cannot exceed 60 mm without great cost and difficulty, especially in sealed tubes. There are also practical. difficulties that can make their inclusion in low-cost, short-duration flight development programs problematic (in particular, the acquisition of quality MCPs, especially in the larger formats, and the yield of sealed tube fabrication cycles). High voltage stability and arcing is a chronic concern, especially with proximity-focused photocathodes. While large-format sealed tube MCP detectors have been made (e.g., 65 mm diameter for GALEX), their fabrication is extraordinarily challenging and not readily duplicated.
Electron Bombarded CCDs (EBCCDs) were developed to overcome the limitations of MCPs. They eliminate the MCP and instead accelerate the photoelectrons from the photocathode directly into the back surface of a CCD. In a typical. EBCCD, photoelectrons are accelerated by ˜10-20 keV and a permanent magnet is used to bend the electrons away from the light path and into the back-illuminated CCD anode (see FIG. 2). Energetic photoelectrons penetrate the surface electrode of the back-illuminated CCD and enter the low-doped sensitive region, where each incident photoelectron liberates many signal. electrons. Since the input-referred noise floor of a moderate-speed CCD is on the order of 5 to 25 electrons, modest multiplication is required in order to allow the EBCCD to be single-photon counting at near UV wavelengths. Moreover, because silicon detectors are sensitive to background radiation at visible and near infrared wavelengths, a magnetic or electrostatic field is used to bend the photoelectrons around a light shield so that there is no direct line-of-sight between the photocathode and CCD. (See, Lowrance, et. al., Photoelectronic Image Devices, 1991, ed. B. L. Morgan, Institute of Physics Conference Series No. 121, the disclosure of which is incorporated herein by reference.) Stray visible light reflected off the photocathode is absorbed by the light shield.
EBCCDs have the highest QE of the currently available detectors/imagers (40% vs. 20% for MCP-based detectors and 5% for CCD/Woods filler combination—all at 120 nm). (See, C. Joseph, Proc. SPIE 3764, 246 (1999), the disclosure of which is incorporated herein by reference.) EBCCDs also benefit from the mature and high quality silicon imaging technology, they are solar blind, they can photon count, and they are, in concept, simpler than MCP-based detectors. However, because conventional. EBCCD technology would merely replace the bulky, expensive, fragile, high voltage MCP-phosphor system with a system that is also bulky, expensive, fragile, and requires very high voltage, conventional. EBCCD systems have not realized their full promise and have found rather limited application. In short, while EBCCDs arrays are potentially very effective solar blind photon counting detectors, their use has been limited due to two major issues:                They are bulky due to the high voltage requirements and large permanent magnets; and        They are hard to manufacture due to their highly reactive and therefore unstable photocathodes.        
The first issue (i.e., need for high voltage) stems from the fact that existing EBCCDs cannot detect low-energy electrons because such electrons cannot penetrate their thick back surface electrode. In order to circumvent this limitation, conventional. EBCCDs accelerate photogenerated electrons to more than 10-15 keV. At these energies, electrons impinging on existing EBCCDs carry sufficient energy to create damaging x rays. The second issue, involving the chemical. reactivity and instability of UV photocathodes, has to be addressed by the use of better photocathode.
Accordingly, a need exists to develop a highly efficient, compact, low mass imager capable of operating as an Electron Bombarded Array (EBA, either CCD or CMOS), Night Vision system, or Image Tube.